Comparison of lightning activity in the two most  active areas of the Congo Basin

Abstract. A comparison of the lightning activity in the two most active areas (Area_max for the main maximum and Area_sec for the secondary maximum) of the Congo Basin is made with data obtained by the World Wide Lightning Location Network (WWLLN) during 2012 and 2013. Both areas of same size (5°  ×  5°) exhibit flash counts in a ratio of about 1.32 for both years and very different distributions of the flash rate density (FRD) with maximums in a ratio of 1.94 and 2.59 for 2012 and 2013, respectively. The FRD is much more widely distributed in Area_sec, which means the whole area contributes more or less equal to the lightning activity. The diurnal cycle is much more pronounced in Area_max than in Area_sec with a ratio between the maximum and the minimum of 15.4 and 4.7, respectively. However, the minimum and maximum of the hourly flash rates are observed roughly at the same time in both areas, between 07:00 and 09:00 UTC and between 16:00 and 17:00 UTC, respectively. In Area_sec the proportion of days with low lightning rate (0–1000 flashes per day) is much larger (∼  45 % in 2013) compared to Area_max (∼  23 % in 2013). In Area_max the proportion of days with moderate lightning rate (1001–6000 flashes per day) is larger (∼  68.5 % in 2013) compared to Area_sec (∼  46 % in 2013). The very intense convective events are slightly more numerous in Area_sec. In summary, the thunderstorm activity in Area_sec is more variable at different scales of time (annually and daily), in intensity and in location. Area_max combines two favourable effects for thunderstorm development, the convergence associated with the African easterly jet of the Southern Hemisphere (AEJ-S) and a geographic effect due to the orography and the presence of a lake. The location of the strong convection in Area_sec is modulated by the distance of westward propagation/regeneration of mesoscale convective systems (MCSs) in relation to the phase of Kelvin waves.

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Lake-effect rains over Lake Victoria and their association with Mesoscale Convective Systems

Journal of Hydrometeorology ◽

10.1175/jhm-d-20-0244.1 ◽

2021 ◽

Author(s):

Sharon E. Nicholson ◽

Douglas Klotter ◽

Adam T. Hartman

Keyword(s):

Large Scale ◽

Lake Victoria ◽

Ground Truth ◽

Mesoscale Convective Systems ◽

Convective Systems ◽

Lake Effect ◽

Mesoscale Convective ◽

Strong Convection ◽

Short Rains ◽

Lake Catchment

AbstractThis article examined rainfall enhancement over Lake Victoria. Estimates of over-lake rainfall were compared with rainfall in the surrounding lake catchment. Four satellite products were initially tested against estimates based on gauges or water balance models. These included TRMM 3B43, IMERG V06 Final Run (IMERG-F), CHIRPS2, and PERSIANN-CDR. There was agreement among the satellite products for catchment rainfall but a large disparity among them for over-lake rainfall. IMERG-F was clearly an outlier, exceeding the estimate from TRMM 3B43 by 36%. The overestimation by IMERG-F was likely related to passive microwave assessments of strong convection, such as prevails over Lake Victoria. Overall, TRMM 3B43 showed the best agreement with the "ground truth" and was used in further analyses. Over-lake rainfall was found to be enhanced compared to catchment rainfall in all months. During the March-to-May long rains the enhancement varied between 40% and 50%. During the October-to-December short rains the enhancement varied between 33% and 44%. Even during the two dry seasons the enhancement was at least 20% and over 50% in some months. While the magnitude of enhancement varied from month to month, the seasonal cycle was essentially the same for over-lake and catchment rainfall, suggesting that the dominant influence on over-lake rainfall is the large-scale environment. The association with Mesoscale Convective Systems (MCSs) was also evaluated. The similarity of the spatial patterns of rainfall and MCS count each month suggested that these produced a major share of rainfall over the lake. Similarity in interannual variability further supported this conclusion.

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Synoptic Wave Perturbations and Convective Systems over Equatorial Africa

Journal of Climate ◽

10.1175/2008jcli2409.1 ◽

2008 ◽

Vol 21 (23) ◽

pp. 6372-6388 ◽

Cited By ~ 51

Author(s):

Hanh Nguyen ◽

Jean-Philippe Duvel

Keyword(s):

Amazon Basin ◽

Spectral Characteristics ◽

Congo Basin ◽

Mesoscale Convective Systems ◽

Kelvin Wave ◽

Equatorial Atlantic ◽

Periodic Behavior ◽

Convective Systems ◽

Equatorial Africa ◽

Mesoscale Convective

Abstract Spectral analysis of the outgoing longwave radiation (OLR) time series over equatorial Africa reveals large oscillations of the convection with periods of between 3 and 6 days. In March and April, when the intertropical convergence zone (ITCZ) migrates northward and crosses equatorial Africa, this periodic behavior is most pronounced with a marked peak at 5–6 days. Robust horizontal and vertical patterns, consistent with a convectively coupled Kelvin wave, can be extracted by a simple composite technique based only on the phase of the convective oscillations over equatorial Africa. The composite reveals differences between continental and adjacent oceanic regions. Over the continent, the stronger oscillation of the convection is associated with larger temperature and moisture anomalies near the surface, suggesting an influence of diabatic processes on the amplitude of the perturbations. Some convective events over equatorial Africa are triggered by waves propagating eastward over the equatorial Atlantic. However, this cannot explain the robust periodic behavior observed over equatorial Africa because the convective variability over the Amazon basin and the equatorial Atlantic have different spectral characteristics with no marked peak at 5–6 days in March and April. The mesoscale convective systems embedded in these synoptic disturbances are studied using satellite brightness temperature at higher spatial (0.5°) and temporal (3 h) resolution than the OLR (respectively, 2.5° and daily average). The diurnal and the wave modulations of occurrence, size, and life cycle of the mesoscale convective systems are inspected. These systems are generated preferentially over the western slopes of the Rift Valley highlands. They propagate west-southwestward over the Congo basin where they reach their maximum size. The 5–6-day perturbations do not modify the diurnal triggering of convective systems notably, but the perturbations do modify their development into larger organized convection, especially over the Congo basin. The implication of these results for understanding the physical source of these 5–6-day perturbations is discussed.

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Convective and Slantwise Trajectory Ascent in Convection-Permitting Simulations of Midlatitude Cyclones

Monthly Weather Review ◽

10.1175/mwr-d-16-0112.1 ◽

2016 ◽

Vol 144 (10) ◽

pp. 3961-3976 ◽

Cited By ~ 10

Author(s):

Stephan Rasp ◽

Tobias Selz ◽

George C. Craig

Keyword(s):

Cold Front ◽

Conveyor Belt ◽

Mesoscale Convective Systems ◽

Convective Systems ◽

Mesoscale Convective ◽

Order Of Magnitude ◽

Unstable Layer ◽

Strong Convection ◽

The Mean ◽

Midlatitude Cyclones

Air parcel ascent in midlatitude cyclones driven by latent heat release has been investigated using convection-permitting simulations together with an online trajectory calculation scheme. Three cyclones were simulated to represent different ascent regimes: one continental summer case, which developed strong convection organized along a cold front; one marine winter case representing a slantwise ascending warm conveyor belt; and one autumn case, which contains both ascent types as well as mesoscale convective systems. Distributions of ascent times differ significantly in mean and shape between the convective summertime case and the synoptic wintertime case, with the mean ascent time being one order of magnitude larger for the latter. For the autumn case the distribution is a superposition of both ascent types, which could be separated spatially and temporally in the simulation. In the slowly ascending airstreams a significant portion of the parcels still experienced short phases of convective ascent. These are linked to line convection in the boundary layer for the wintertime case and an elevated conditionally unstable layer in the autumn case. Potential vorticity (PV) modification during ascent has also been investigated. Despite the different ascent characteristics it was found that net PV change between inflow and outflow levels is very close to zero in all cases. The spread of individual PV values, however, is increased after the ascent. This effect is more pronounced for convective trajectories.

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Earlier Seasonal Onset of Intense Mesoscale Convective Systems in the Congo Basin Since 1999

Geophysical Research Letters ◽

10.1029/2018gl080516 ◽

2018 ◽

Vol 45 (24) ◽

Cited By ~ 10

Author(s):

Christopher M. Taylor ◽

Andreas H. Fink ◽

Cornelia Klein ◽

Douglas J. Parker ◽

Françoise Guichard ◽

...

Keyword(s):

Congo Basin ◽

Mesoscale Convective Systems ◽

Convective Systems ◽

Mesoscale Convective

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Mesoscale Convective Systems in Relation to African and Tropical Easterly Jets

Monthly Weather Review ◽

10.1175/mwr-d-13-00247.1 ◽

2014 ◽

Vol 142 (9) ◽

pp. 3224-3242 ◽

Cited By ~ 5

Author(s):

L. Besson ◽

Y. Lemaître

Keyword(s):

Life Cycle ◽

Deep Convection ◽

Mesoscale Convective Systems ◽

Convective Systems ◽

African Easterly Jet ◽

Interaction Processes ◽

Tropical Easterly Jet ◽

Mesoscale Convective ◽

Jet Streaks

This paper documents the interaction processes between mesoscale convective systems (MCS), the tropical easterly jet (TEJ), and the African easterly jet (AEJ) over West Africa during the monsoon peak of 2006 observed during the African Monsoon Multidisciplinary Analyses (AMMA) project. The results highlight the importance of the cloud system localization relative to the jets in order to explain their duration and life cycle. A systematical study reveals that intense and long-lived MCSs correspond to a particular pattern where clouds associated with deep convection are located in entrance regions of TEJ and in exit regions of AEJ. A case study on a particularly well-documented convective event characterizes this link and infers the importance of jet streaks in promoting areas of divergence, favoring the persistence of MCSs.

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Precipitation and Mesoscale Convective Systems: Radiative Impact of Dust over Northern Africa

Monthly Weather Review ◽

10.1175/mwr-d-18-0103.1 ◽

2018 ◽

Vol 146 (9) ◽

pp. 3011-3029 ◽

Cited By ~ 2

Author(s):

Irene Reinares Martínez ◽

Jean-Pierre Chaboureau

Keyword(s):

Convective Available Potential Energy ◽

Dust Emission ◽

Northern Africa ◽

Mesoscale Convective Systems ◽

Lower Atmosphere ◽

Radiative Effect ◽

Near Surface ◽

Convective Systems ◽

African Easterly Jet ◽

Mesoscale Convective

Abstract The radiative effect of dust on precipitation and mesoscale convective systems (MCSs) is examined during a case of dust emission and transport from 9 to 14 June 2006 over northern Africa. The same method to identify and track different cloud types is applied to satellite observations and two convection-permitting simulations (with grid mesh of 2.5 km), with and without the radiative effect of dust, performed with the MesoNH model. The MCSs produce most of the observed total precipitation (66%), and the long-lived systems (lasting 6 h or more) are responsible for 55% of the total. Both simulations reproduce the observed distribution of precipitation between the cloud categories but differ due to the radiative effects of dust. The overall impacts of dust are a warming of the midtroposphere; a cooling of the near surface, primarily in the western parts of northern Africa; and a decrease in precipitation due to a too-low number of long-lived MCSs. The drop in their number is due to the stabilization of the lower atmosphere, which inhibits the triggering of convection. The long-lived MCSs are a little longer lived, faster, and more efficient in rainfall production when accounting for the dust–radiation interaction. This higher degree of organization is due to the larger convective available potential energy and an intensified African easterly jet. The latter is, in turn, a response to the variation in the meridional gradient of the temperature induced by the dust.

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Dry soils can intensify mesoscale convective systems

Proceedings of the National Academy of Sciences ◽

10.1073/pnas.2007998117 ◽

2020 ◽

Vol 117 (35) ◽

pp. 21132-21137 ◽

Cited By ~ 2

Author(s):

Cornelia Klein ◽

Christopher M. Taylor

Keyword(s):

Soil Moisture ◽

Positive Impact ◽

Surface Fluxes ◽

Mesoscale Convective Systems ◽

Convective Systems ◽

The World ◽

Mesoscale Convective ◽

Moisture State ◽

Favorable Conditions ◽

The Impact

Soil moisture can feed back on rainfall through the impact of surface fluxes on the environment in which convection develops. The vast majority of previous research has focused on the initiation of convection, but in many regions of the world, the majority of rain comes from remotely triggered mesoscale convective systems (MCSs). Here we conduct a systematic observational analysis of soil moisture feedbacks on propagating MCSs anywhere in the world and show a strong positive impact of drier soils on convection within mature MCSs. From thousands of storms captured in satellite imagery over the Sahel, we find that convective cores within MCSs are favored on the downstream side of dry patches ≥200 km across. The effect is particularly strong during the afternoon–evening transition when convection reaches its diurnal peak in intensity and frequency, with dry soils accounting for an additional one in five convective cores. Dry soil patterns intensify MCSs through a combination of convergence, increased instability, and wind shear, all factors that strengthen organized convection. These favorable conditions tend to occur in the vicinity of a surface-induced anomalous displacement of the Sahelian dry line/intertropical discontinuity, suggesting a strong link between dry line dynamics and soil moisture state. Our results have important implications for nowcasting of severe weather in the Sahel and potentially in other MCS hotspot regions of the world.

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Modeling the Flash Rate of Thunderstorms. Part I: Framework

Monthly Weather Review ◽

10.1175/mwr-d-10-05031.1 ◽

2011 ◽

Vol 139 (10) ◽

pp. 3093-3111 ◽

Cited By ~ 15

Author(s):

Johannes M. L. Dahl ◽

Hartmut Höller ◽

Ulrich Schumann

Keyword(s):

Radar Data ◽

Mesoscale Convective Systems ◽

New Approach ◽

Flash Rate ◽

Convective Systems ◽

Cloud Properties ◽

Lightning Detection ◽

Mesoscale Convective ◽

Lightning Discharges ◽

New Framework

Abstract In this study a straightforward theoretical approach to determining the flash rate in thunderstorms is presented. A two-plate capacitor represents the basic dipole charge structure of a thunderstorm, which is charged by the generator current and discharged by lightning. If the geometry of the capacitor plates, the generator-current density, and the lightning charge are known, and if charging and discharging are in equilibrium, then the flash rate is uniquely determined. To diagnose the flash rate of real-world thunderstorms using this framework, estimates of the required relationships between the predictor variables and observable cloud properties are provided. With these estimates, the flash rate can be parameterized. In previous approaches, the lightning rate has been set linearly proportional to the electrification rate (such as the storm’s generator power or generator current), which implies a constant amount of neutralization by lightning discharges (such as lightning energy or lightning charge). This leads to inconsistencies between these approaches. Within the new framework proposed here, the discharge strength is allowed to vary with storm geometry, which remedies the physical inconsistencies of the previous approaches. The new parameterization is compared with observations using polarimetric radar data and measurements from the lightning detection network, LINET. The flash rates of a broad spectrum of discrete thunderstorm cells are accurately diagnosed by the new approach, while the flash rates of mesoscale convective systems are overestimated.

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